Field effect device with oxide semiconductor layer

ABSTRACT

Provided is a semiconductor device in which generation of a parasitic channel in an end region of an oxide semiconductor film is suppressed. The semiconductor device includes a gate electrode, an oxide semiconductor film, a source electrode and a drain electrode, and a channel region formed in the oxide semiconductor film. The channel region is formed between a first side surface of the source electrode and a second side surface of the drain electrode opposite to the first side surface. The oxide semiconductor film has an end region which does not overlap with the gate electrode. The end region which does not overlap with the gate electrode is positioned between a first region that is the nearest to one end of the first side surface and a second region that is the nearest to one end of the second side surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including an oxide semiconductor. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as an EL display device.

2. Description of the Related Art

A technique by which a transistor is formed using a semiconductor thin film formed over a substrate has attracted attention. The transistor is applied to a wide range of electronic devices such as a liquid crystal display device. A silicon-based semiconductor material has been widely known as a material for a semiconductor thin film applicable to a transistor. Besides, an oxide semiconductor has attracted attention.

FIG. 13 is a top view of a bottom-gate transistor in which In—Ga—Zn—O (IGZO) is used as an oxide semiconductor. This transistor includes a gate insulating film (not shown) formed over a gate electrode 1001, an oxide semiconductor film 1002 which is formed using IGZO over the gate insulating film, and a source electrode 1003 and a drain electrode 1004 which are formed over the oxide semiconductor film 1002 (for example, see Patent Document 1).

FIG. 14 shows Id (drain current)−Vg (gate voltage) curves of the transistor shown in FIG. 13 which are obtained before and after GBT (Gate Bias Temperature) stress test. The broken line shows the Id−Vg curve before the GBT stress test and the solid line shows the Id−Vg curve after the GBT stress test.

In the transistor shown in FIG. 13, because an entire end region of the oxide semiconductor film 1002 overlaps with the gate electrode 1001, stress is generated in the end region of the oxide semiconductor film 1002 due to an electric field by application of negative voltage to the gate electrode 1001 when the GBT stress test is performed. Thus, the end region of the oxide semiconductor film 1002 is easily made to have n-type conductivity. Accordingly, a parasitic channel is generated in the end region of the oxide semiconductor film 1002 and a leakage path is formed between the source electrode 1003 and the drain electrode 1004, which results in a hump in the Id−Vg curve as shown in FIG. 14. In the circuit using the above transistor, the generation of the hump has adverse effects that voltage in the circuit cannot be retained, current consumption is increased, and the like. Note that “hump” means that the Id increases in the Id−Vg curve in stages.

The reason that the end region of the oxide semiconductor film 1002 becomes n-type is as follows. The oxide semiconductor film 1002 is activated by generation of electric field stress in the oxide semiconductor film 1002, and its reactivity is enhanced. In particular, when the oxide semiconductor film 1002 is formed using a photolithography process and an etching process in manufacturing the transistor, a side surface of the end region of the oxide semiconductor film 1002 is directly exposed to an etching atmosphere such as plasma although a top surface of the oxide semiconductor film 1002 is protected by a photoresist layer. Thus, the side surface of the oxide semiconductor film 1002 is damaged in the process and is easily contaminated by an impurity. As a result, the end region of the oxide semiconductor film 1002 is more easily activated compared to the other portion of the oxide semiconductor film 1002, thereby being likely to become n-type by the GBT stress test.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2007-299913

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide a semiconductor device in which generation of a parasitic channel in an end region of an oxide semiconductor film that is an active layer is suppressed.

One embodiment of the present invention is a semiconductor device including a gate electrode, an oxide semiconductor film, a source electrode and a drain electrode, and a channel region formed in the oxide semiconductor film. The channel region is formed between a first side surface of the source electrode and a second side surface of the drain electrode which is opposite to the first side surface. The oxide semiconductor film has an end region which does not overlap with the gate electrode and which is positioned between a first region that is the nearest to one end of the first side surface and a second region that is the nearest to one end of the second side surface.

According to the one embodiment of the present invention, the oxide semiconductor film has the end region which does not overlap with the gate electrode, and the end region which does not overlap with the gate electrode is positioned between the first region that is the nearest to the one end of the first side surface and the second region that is the nearest to the one end of the second side surface, whereby generation of a parasitic channel in the end region of the oxide semiconductor film can be suppressed even when voltage is applied to the gate electrode.

Another embodiment of the present invention is a semiconductor device including a gate electrode, a gate insulating film formed over the gate electrode, an oxide semiconductor film formed over the gate insulating film, a source electrode and a drain electrode formed so as to be in contact with the oxide semiconductor film, and a channel region formed in the oxide semiconductor film. The channel region is formed between a first side surface of the source electrode and a second side surface of the drain electrode which is opposite to the first side surface. The oxide semiconductor film has an end region which does not overlap with the gate electrode and which is positioned between a first region that is the nearest to one end of the first side surface and a second region that is the nearest to one end of the second side surface.

In one embodiment of the present invention, at least one of the source electrode and the drain electrode and the gate electrode are each preferably arranged so as to extend from one side of the oxide semiconductor film, and the at least one of the source electrode and the drain electrode preferably overlaps with both the oxide semiconductor film and the gate electrode.

In one embodiment of the present invention, the end region of the oxide semiconductor film which does not overlap with the gate electrode is preferably positioned between a third region that is the nearest to the other end of the first side surface and a fourth region that is the nearest to the other end of the second side surface.

In one embodiment of the present invention, the end region of the oxide semiconductor film which does not overlap with the gate electrode is preferably a continuous region from the first region to the second region.

In one embodiment of the present invention, the end region of the oxide semiconductor film which does not overlap with the gate electrode is preferably a continuous region from the third region to the fourth region.

In one embodiment of the present invention, the end region of the oxide semiconductor film which does not overlap with the gate electrode is preferably provided outside a space between the first side surface and the second side surface.

In one embodiment of the present invention, the oxide semiconductor film preferably has an end region which overlaps with the gate electrode, and the end region of the oxide semiconductor film which overlaps with the gate electrode preferably entirely overlaps with at least one of the source electrode and the drain electrode.

According to one embodiment of the present invention, the oxide semiconductor film is preferably a film containing an oxide of one or more elements selected from indium, zinc, gallium, and tin.

According to one embodiment of the present invention, the oxide semiconductor film preferably includes a crystal portion where a c-axis is aligned in a direction parallel to a normal vector of a surface where the oxide semiconductor film is formed.

A semiconductor device in which generation of a parasitic channel in an end region of an oxide semiconductor film that is an active layer is suppressed can be provided by application of one embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 1B is a cross-sectional view taken along line A-B in FIG. 1A; and FIG. 1C is a cross-sectional view taken along line C-D in FIG. 1A;

FIG. 2 is a perspective view of the semiconductor device shown in FIG. 1A, which is seen in the direction of arrow 149;

FIG. 3 is a plan view of a semiconductor device according to another embodiment of the present invention;

FIGS. 4A to 4C are cross-sectional views illustrating a method for manufacturing the semiconductor device shown in FIGS. 1A to 1C;

FIG. 5A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 5B is a cross-sectional view taken along line A-B in FIG. 5A; and FIG. 5C is a cross-sectional view taken along line C-D in FIG. 5A;

FIG. 6A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 6B is a cross-sectional view taken along line A-B in FIG. 6A; and FIG. 6C is a cross-sectional view taken along line C-D in FIG. 6A;

FIG. 7A is a plan view of a transistor that is a simulation object; FIG. 7B is a cross-sectional view taken along line A-A′ in FIG. 7A; and FIG. 7C is a cross-sectional view taken along line C-C′ in FIG. 7A;

FIG. 8A shows results of a first simulation and FIG. 8B shows results of a second simulation;

FIG. 9 is a top view of a pixel of a liquid crystal display device according to one embodiment of the present invention;

FIG. 10 is a circuit diagram of the pixel of the liquid crystal display device shown in FIG. 9;

FIG. 11 is a top view of a pixel of an EL display device according to one embodiment of the present invention;

FIG. 12 is a circuit diagram of the pixel of the EL display device shown in FIG. 11;

FIG. 13 is a top view of a bottom gate transistor in which In—Ga—Zn—O (IGZO) is used as an oxide semiconductor; and

FIG. 14 shows Id−Vg curves that are the results of the GBT stress test conducted on the transistor shown in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Therefore, the invention should not be construed as being limited to the description in the following embodiments.

Note that both a source electrode and a drain electrode in a transistor are connected to an oxide semiconductor film. Current flows in accordance with a potential difference between the source electrode and the drain electrode when voltage is applied to a gate electrode; therefore, the source electrode and the drain electrode may be interchanged depending on an operation, and in that case it may be difficult to identify the positions of the source electrode and the drain electrode. Thus, when a structure of the transistor is described, a source electrode and a drain electrode may be interchanged.

Embodiment 1 Bottom Gate Top Contact-Channel Etch Structure in which the Width of a Gate Electrode is Larger than the Width of Each of a Source Electrode and a Drain Electrode

FIG. 1A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 1B is a cross-sectional view taken along line A-B in FIG. 1A; and FIG. 1C is a cross-sectional view taken along line C-D in FIG. 1A. FIG. 2 is a perspective view of the semiconductor device shown in FIG. 1A, which is seen from the direction of arrow 149.

The semiconductor device includes a transistor 150 having a bottom-gate structure (also referred to as an inverted staggered structure). Note that in FIG. 1A, some components of the transistor 150 (e.g., a gate insulating film 108 and the like) are not illustrated for simplification.

As illustrated in FIGS. 1B and 1C, a base insulating film 104 is formed over a substrate 102, and a gate electrode 106 is formed over the base insulating film 104. The gate insulating film 108 is formed over the base insulating film 104 and the gate electrode 106. An oxide semiconductor film 110 having a channel region is formed over the gate insulating film 108. In Embodiment 1, the oxide semiconductor film 110 is an oxide semiconductor film using an IGZO.

The oxide semiconductor film 110 is provided in a position that is in contact with the gate insulating film 108 and overlaps with the gate electrode 106. A source electrode 114 a and a drain electrode 114 b are formed over the oxide semiconductor film 110 and the gate insulating film 108. The source electrode 114 a and the drain electrode 114 b are electrically connected to the oxide semiconductor film 110. Note that the source electrode 114 a and the drain electrode 114 b that are electrically connected to the oxide semiconductor film 110 may be provided between the oxide semiconductor film 110 and the gate insulating film 108.

The channel region of the oxide semiconductor film 110 is formed between a first side surface 214 a of the source electrode 114 a and a second side surface 214 b of the drain electrode 114 b which is opposite to the first side surface 214 a. The oxide semiconductor film 110 has an end region 206 which does not overlap with the gate electrode 106.

Note that in this specification, an end region of the oxide semiconductor film means a side surface of the oxide semiconductor film, where process damage, impurity contamination, or the like due to the etching of the oxide semiconductor film is caused.

The end region of the oxide semiconductor film 110 shown in FIG. 1A includes the end region 206 which does not overlap with the gate electrode 106 and an end region which overlaps with the gate electrode 106. The end region which overlaps with the gate electrode 106 is a region where the oxide semiconductor film 110 having a quadrangular planar shape overlaps with the gate electrode 106 arranged so as to extend from one side of the quadrangle and which is a region formed on the one side of the quadrangle (see FIG. 1A). Further, the end region 206 which does not overlap with the gate electrode 106 is a region which is formed on three sides (all sides excluding the side where the end region which overlaps with the gate electrode 106 is formed) of the quadrangle of the planar shape of the oxide semiconductor film 110.

As shown in FIG. 1A, the end region 206 of the oxide semiconductor film 110 which does not overlap with the gate electrode 106 includes a region located between a first region 206 a that is the nearest to one end 314 a of the first side surface 214 a and a second region 206 b that is the nearest to one end 314 b of the second side surface 214 b. Note that the end region 206 of the oxide semiconductor film 110 which does not overlap with the gate electrode 106 is preferably a continuous region from the first region 206 a to the second region 206 b.

Further, as shown in FIG. 1A and FIG. 2, the end region 206 which does not overlap with the gate electrode 106 includes a region located between a third region 206 c that is the nearest to the other end 414 a of the first side surface 214 a and a fourth region 206 d that is the nearest to the other end 414 b of the second side surface 214 b. Note that the end region 206 of the oxide semiconductor film 110 which does not overlap with the gate electrode 106 is preferably a continuous region from the third region 206 c to the fourth region 206 d.

Note that the first to the fourth regions 206 a to 206 d represent linear regions that are on the side surfaces of the oxide semiconductor film 110 (see FIG. 2)

In this manner, the transistor 150 having a bottom-gate structure is formed over the substrate 102. Further, an interlayer insulating film 116 and a planarization insulating film (not shown) may be provided over the transistor 150. In detail, the interlayer insulating film 116 is provided over the source electrode 114 a and the drain electrode 114 b, and the planarization insulating film is provided over the interlayer insulating film 116.

According to one embodiment of the present invention shown in FIGS. 1A to 1C and FIG. 2, the gate electrode 106 is arranged so as to extend from the one side of the quadrangle of the oxide semiconductor film 110, and the source electrode 114 a and the drain electrode 114 b are arranged so as to extend in the same direction as the gate electrode 106, so that the end region 206 which does not overlap with the gate electrode 106 can be formed on three sides of the quadrangle of the planar shape of the oxide semiconductor film 110. The end region 206 includes a region located between the first region 206 a and the second region 206 b and a region located between the third region 206 c and the fourth region 206 d. Thus, stress due to an electric field is not generated in the end region of the oxide semiconductor film 110 which does not overlap with the gate electrode 106, even when voltage is applied to the gate electrode 106; accordingly, the end region 206 of the oxide semiconductor film 110 is easily made to have an n-type conductivity. As a result, a leakage path due to current flowing through the one end 314 a of the first side surface 214 a of the source electrode 114 a, the first region 206 a of the end region 206 which is the nearest to the one end 314 a, the second region 206 b of the end region 206 which is the nearest to the one end 314 b, and the one end 314 b of the second side surface 214 b of the drain electrode 114 b is not formed. Similarly, a leakage path due current flowing through the other end 414 a of the first side surface 214 a of the source electrode 114 a, the third region 206 c of the end region 206 which is the nearest to the other end 414 a, the fourth region 206 d of the end region 206 which is the nearest to the other end 414 b, and the other end 414 b of the second side surface 214 b of the drain electrode 114 b is not formed. Thus, generation of a parasitic channel in the end region 206 can be suppressed, so that formation of a parasitic transistor can be prevented. Further, with the use of the transistor, the adverse effects that voltage in the current cannot be retained, current consumption is increased, and the like can be reduced.

FIG. 3 is a plan view showing a semiconductor device according to another embodiment of the present invention. In the semiconductor device, the end region 206 positioned between the first region 206 a and the second region 206 b and the end region 206 positioned between the third region 206 c and the fourth region 206 d are only end regions which do not overlap with the gate electrode 106, and the other end regions overlap with the gate electrode 106.

According to another embodiment of the present invention, even when voltage is applied to the gate electrode 106, stress due to an electric field is not generated in the end regions of the oxide semiconductor film 110 which do not overlap with the gate electrode 106, so that the end region 206 of the oxide semiconductor film 110 is not easily made to have an n-type conductivity. Thus, advantageous effects similar to those of the embodiment of the present invention shown in FIGS. 1A to 1C and FIG. 2 can be obtained.

Further, as shown in FIGS. 1A to 1C and FIG. 3, it is preferable that the end region 206 of the oxide semiconductor film 110 which does not overlap with the gate electrode 106 be not positioned between the first side surface 214 a of the source electrode 114 a and the second side surface 214 b of the drain electrode 114 b. In other words, the end region 206 which does not overlap with the gate electrode 106 is preferably provided outside the space between the first side surface 214 a and the second side surface 214 b. This is because if the end region 206 which is positioned between the first side surface 214 a and the second side surface 214 b and which does not overlap with the gate electrode 106 is close to the first side surface 214 a and the second side surface 214 b, a leakage path is formed in the end region and this results in generation of a parasitic channel.

[Detailed Description of Oxide Semiconductor Film 110]

Note that the oxide semiconductor film 110 may have an amorphous structure or a polycrystalline structure. Alternatively, the oxide semiconductor film 110 may be a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification and the like, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal portions in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film).

Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation.

With use of the CAAC-OS film in a transistor, change in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Change and variation in threshold voltages can be suppressed. Thus, the transistor has high reliability.

In a crystal part or a crystalline oxide semiconductor film, defects in the bulk can be further reduced. Further, when the surface flatness of the crystal part or the crystalline oxide semiconductor film is enhanced, a transistor including the oxide semiconductor film can have higher field-effect mobility than a transistor including an amorphous oxide semiconductor film. In order to improve the surface flatness of the oxide semiconductor film, the oxide semiconductor film is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (R_(a)) of less than or equal to 0.15 nm, preferably less than or equal to 0.1 nm.

Note that, Ra is obtained by three-dimension expansion of center line average roughness that is defined by JIS B0601 so as to be applied to a plane. The Ra can be expressed as an “average value of the absolute values of deviations from a reference surface to a designated surface” and is defined by the formula below.

$\begin{matrix} {{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0\;}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

In Formula 1, S₀ represents an area of a plane to be measured (a rectangular region which is defined by four points represented by coordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀ represents an average height of the plane to be measured. Ra can be measured using an atomic force microscope (AFM).

For the oxide semiconductor film, an oxide semiconductor having a wider band gap than that of silicon, i.e., 1.1 eV, is preferably used. For example, an In—Ga—Zn-based metal oxide having a band gap of about 3.15 eV, indium oxide having a band gap of about 3.0 eV, indium tin oxide having a band gap of about 3.0 eV, indium gallium oxide having a band gap of about 3.3 eV, indium zinc oxide having a band gap of about 2.7 eV, tin oxide having a band gap of about 3.3 eV, zinc oxide having a band gap of about 3.37 eV, or the like can be preferably used. With the use of such a material, the off-state current of the transistor can be kept extremely low.

An oxide semiconductor used for the oxide semiconductor film preferably contains at least one selected from the group consisting of indium (In), zinc (Zn), and gallium (Ga). In particular, In and Zn are preferably contained. As a stabilizer for reducing a variation in electrical characteristics among transistors including the oxide semiconductor, tin (Sn) is preferably contained.

For example, as the oxide semiconductor, any of the following can be used: indium oxide; tin oxide; zinc oxide; a two-component metal oxide such as an In—Zn-based metal oxide, a Sn—Zn-based metal oxide, or In—Ga-based metal oxide; a three-component metal oxide such as an In—Ga—Zn-based metal oxide (also referred to as IGZO), In—Sn—Zn-based metal oxide, Sn—Ga—Zn-based metal oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based metal oxide.

Here, an In—Ga—Zn-based metal oxide means a metal oxide whose main components are In, Ga, and Zn, and there is no particular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 is satisfied, and m is not an integer) may be used as an oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co, or the above-described element as a stabilizer. Alternatively, as the oxide semiconductor, a material represented by a chemical formula, In₂SnO₅(ZnO), (n>0, n is a natural number) may be used.

For example, an In—Ga—Zn-based metal oxide with an atomic ratio where In:Ga:Zn=1:1:1, In:Ga:Zn=3:1:2, or In:Ga:Zn=2:1:3, or an oxide whose atomic ratio is in the neighborhood of the above compositions may be used.

It is preferable that an impurity such as hydrogen or water be contained in the oxide semiconductor film as little as possible in the formation step of the oxide semiconductor film. For example, it is preferable that the substrate be preheated in a preheating chamber of a sputtering apparatus as pretreatment for formation of the oxide semiconductor film so that an impurity such as hydrogen or water adsorbed to the substrate is eliminated and removed. Then, the oxide semiconductor film is preferably formed in a film formation chamber from which remaining water is removed.

In order to remove water in the preheating chamber and the film formation chamber, an entrapment vacuum pump, for example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. Further, an evacuation unit may be a turbo pump provided with a cold trap. From the preheating chamber and the film formation chamber which is evacuated with a cryopump, a hydrogen atom, a compound containing a hydrogen atom such as water (H₂O) (preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of an impurity such as hydrogen or water in the oxide semiconductor film can be reduced.

Note that an In—Ga—Zn-based metal oxide film is formed as the oxide semiconductor film by a sputtering method. The oxide semiconductor film can be formed by a sputtering method in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen.

As a target used for forming an In—Ga—Zn-based metal oxide film as the oxide semiconductor film by a sputtering method, for example, a metal oxide target with an atomic ratio where In:Ga:Zn=1:1:1, a metal oxide target with an atomic ratio where In:Ga:Zn=3:1:2, or a metal oxide target with an atomic ratio where In:Ga:Zn=2:1:3 can be used. However, a material and composition of a target used for formation of the oxide semiconductor film is not limited to the above.

Further, when the oxide semiconductor film is formed using the above metal oxide target, the composition of the target is different from the composition of a film formed over the substrate in some cases. For example, when the metal oxide target having a molar ratio where In₂O₃:Ga₂O₃:ZnO=1:1:1 is used, the composition of the thin oxide semiconductor film becomes In₂O₃:Ga₂O₃:ZnO=1:1:0.6 to 1:1:0.8 [molar ratio] in some cases, though it depends on the film formation conditions. This is because in formation of the oxide semiconductor film, ZnO is sublimed, or because a sputtering rate differs between the components of In₂O₃, Ga₂O₃, and ZnO.

Accordingly, when a thin film having a preferable composition ratio is formed, a composition ratio of the metal oxide target needs to be adjusted in advance. For example, in order to make the composition ratio of the thin oxide semiconductor film be In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio], the composition ratio of the metal oxide target is made to be In₂O₃:Ga₂O₃:ZnO=1:1:1.5 [molar ratio], for instance. In other words, the ZnO content of the metal oxide target is made higher in advance. The composition ratio of the target is not limited to the above value, and can be adjusted as appropriate depending on the film formation conditions or the composition of the thin film to be formed. Further, it is preferable to increase the ZnO content of the metal oxide target because in that case, the crystallinity of the obtained thin film is improved.

The relative density of the metal oxide target is 90% to 100% inclusive, preferably, 95% to 99.9% inclusive. By using the metal oxide target with high relative density, a dense oxide semiconductor film can be formed.

As a sputtering gas used for forming the oxide semiconductor film, it is preferable to use a high-purity gas from which impurities such as hydrogen or water are removed.

There are three methods for forming a CAAC-OS film when the CAAC-OS film is used as the oxide semiconductor film. The first method is to form an oxide semiconductor film at a temperature higher than or equal to 200° C. and lower than or equal to 450° C. to form, in the oxide semiconductor film, crystal parts in which the c-axes are aligned in the direction parallel to a normal vector of a surface where the oxide semiconductor film is formed or a normal vector of a surface of the oxide semiconductor film. The second method is to form an oxide semiconductor film with a small thickness and then heat it at a temperature higher than or equal to 200° C. and lower than or equal to 700° C., to form, in the oxide semiconductor film, crystal parts in which the c-axes are aligned in the direction parallel to a normal vector of a surface where the oxide semiconductor film is formed or a normal vector of a surface of the oxide semiconductor film. The third method is to form a first oxide semiconductor film with a small thickness, then heat it at a temperature higher than or equal to 200° C. and lower than or equal to 700° C., and form a second oxide semiconductor film, to form, in the oxide semiconductor film, crystal parts in which the c-axes are aligned in the direction parallel to a normal vector of a surface where the oxide semiconductor film is formed or a normal vector of a surface of the oxide semiconductor film.

By heating the substrate during the film formation, the concentration of an impurity such as hydrogen or water in the oxide semiconductor film formed can be reduced. In addition, damage by sputtering can be reduced, which is preferable. The oxide semiconductor film may be formed by an ALD (atomic layer deposition) method, an evaporation method, a coating method, or the like.

Note that when a crystalline (single-crystal or microcrystalline) oxide semiconductor film other than a CAAC-OS film is formed as the oxide semiconductor film, the film formation temperature and the temperature of heat treatment are not particularly limited.

As a method for processing the oxide semiconductor film, a wet etching method or a dry etching method can be used to etch the oxide semiconductor film. An etching gas such as BCl₃, Cl₂, or O₂ can be used in the dry etching method. Further, a dry etching apparatus using a high-density plasma source such as electron cyclotron resonance (ECR) or inductive coupled plasma (ICP) can be used to improve an etching rate.

After the oxide semiconductor film is formed, the oxide semiconductor film may be subjected to heat treatment. The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. Through the heat treatment, excess hydrogen, water, and the like which are contained in the oxide semiconductor film can be removed. Note that the heat treatment is also referred to as dehydration treatment (dehydrogenation treatment) in this specification and the like in some cases.

The heat treatment can be performed in such a manner that, for example, an object to be processed is introduced into an electric furnace in which a resistance heater or the like is used and heated at 450° C. in a nitrogen atmosphere for one hour. The oxide semiconductor film is not exposed to the air during the heat treatment so that entry of an impurity such as hydrogen or water can be prevented.

Note that a heat treatment apparatus is not limited to an electric furnace, and may include a device for heating an object to be processed by heat conduction or heat radiation from a medium such as a heated gas. For example, an RTA (rapid thermal annealing) apparatus such as a GRTA (gas rapid thermal annealing) apparatus, or an LRTA (lamp rapid thermal annealing) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used.

For example, as the heat treatment, GRTA treatment may be performed as follows. An object to be processed is put in an inert gas atmosphere that has been heated, heated for several minutes, and taken out from the inert gas atmosphere. The GRTA process enables high-temperature heat treatment for a short time. Moreover, the GRTA process can be employed even when the temperature exceeds the upper temperature limit of the object. Note that the inert gas may be switched to a gas including oxygen during the process.

Note that as the inert gas atmosphere, an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain hydrogen, water, or the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is set to 6N (99.9999%) or more, preferably 7N (99.99999%) or more (i.e., the impurity concentration is 1 ppm or less, preferably 0.1 ppm or less).

The dehydration treatment (dehydrogenation treatment) might be accompanied by elimination of oxygen which is a main constituent material for an oxide semiconductor film to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy. Therefore, in the case where the dehydration treatment (dehydrogenation treatment) is performed, oxygen is preferably supplied to the oxide semiconductor film. By supply of oxygen to the oxide semiconductor film, an oxygen vacancy in the film can be repaired.

The oxygen vacancy in the second oxide semiconductor film is compensated as follows, for example: after the oxide semiconductor film is subjected to the dehydration treatment (dehydrogenation treatment), a high-purity oxygen gas, a high-purity dinitrogen monoxide gas, or ultra dry air (the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, more preferably less than or equal to 10 ppb, in the measurement with the use of a dew-point instrument of a cavity ring down laser spectroscopy (CRDS) system) is introduced into the same furnace. It is preferable that hydrogen, water, or the like be not contained in the oxygen gas or the dinitrogen monoxide gas. The purity of the oxygen gas or the dinitrogen monoxide gas which is introduced into the heat treatment apparatus is preferably 6N (99.9999%) or more, further preferably 7N (99.99999%) or more (i.e., the impurity concentration in the oxygen gas or the dinitrogen monoxide gas is preferably less than or equal to 1 ppm, further preferably less than or equal to 0.1 ppm).

As an example of a method of supplying oxygen to the oxide semiconductor film, oxygen (including at least any one of oxygen radicals, oxygen atoms, and oxygen ions) is added to the oxide semiconductor film in order to supply oxygen to the oxide semiconductor film. An ion implantation method, an ion doping method, plasma treatment, or the like can be used as a method for adding oxygen.

As another example of a method for supplying oxygen to the oxide semiconductor film, oxygen may be supplied to the oxide semiconductor film in such a manner that the base insulating film, the gate insulating film, or the like is heated and part of oxygen is released.

As described above, after formation of the oxide semiconductor film, it is preferable that dehydration treatment (dehydrogenation treatment) be performed to remove hydrogen, water, or the like from the oxide semiconductor film, so that the oxide semiconductor film is highly purified so as to include as few impurities as possible, and oxygen whose amount is reduced in the dehydration treatment (dehydrogenation treatment) be added to the oxide semiconductor or oxygen be supplied excessively to fill the oxygen vacancies in the oxide semiconductor film. Further, supplying oxygen to the oxide semiconductor film may be expressed as oxygen adding treatment or treatment for making an oxygen-excess state.

In this manner, hydrogen, water, or the like is removed from the oxide semiconductor film by dehydration treatment (dehydrogenation treatment) and oxygen vacancies therein are filled by oxygen adding treatment, whereby the oxide semiconductor film can be turned into an electrically i-type (intrinsic) or substantially i-type oxide semiconductor film. Specifically, the concentration of hydrogen in the oxide semiconductor film is lower than or equal to 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 5×10¹⁷ atoms/cm³. Note that the concentration of hydrogen in the oxide semiconductor film is measured by secondary ion mass spectrometry (SIMS).

The number of carriers generated due to a donor in the oxide semiconductor film, in which hydrogen concentration is reduced to a sufficiently low so that the oxide semiconductor film is purified and in which defect states in a band gap due to oxygen deficiency are reduced by sufficiently supplying oxygen as described above, is very small (close to zero); carrier concentration is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. In a transistor including such an oxide semiconductor film, the off-state current (per unit channel width (1 μm) here) at room temperature (25° C.), for example, is less than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferably less than or equal to 10 zA, more preferably less than or equal to 100 yA (1 yA (yoctoampere) is 1×10⁻²⁴ A). The transistor with very excellent off-state current characteristics can be obtained with the use of such an i-type (intrinsic) or substantially i-type oxide semiconductor.

<Method for Manufacturing Transistor>

Next, a method for manufacturing the semiconductor device illustrated in FIGS. 1A to 1C will be described with reference to FIGS. 4A to 4C. Note that portions similar to those in FIGS. 1A to 1C are denoted by the same reference numerals, and description thereof is skipped.

First, the base insulating film 104 is formed over the substrate 102. Next, after a conductive film is formed over the base insulating film 104, the gate electrode 106 is formed through a photolithography process and an etching process (see FIG. 4A).

For the substrate 102, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used. In the mass production, a mother glass with the following size is preferably used for the substrate 102: the 8-th generation (2160 mm×2460 mm); the 9-th generation (2400 mm×2800 mm, or 2450 mm×3050 mm); the 10-th generation (2950 mm×3400 mm); or the like. A mother glass considerably shrinks when the treatment temperature is high and the treatment time is long. High process temperature and a long period of process time drastically shrink the mother glass. Thus, in the case where mass production is performed with the use of the mother glass, it is preferable that the heat process in the manufacturing process be preferably performed at a temperature lower than or equal to 600° C., further preferably lower than or equal to 450° C., still further preferably lower than or equal to 350° C.

The base insulating film 104 can be formed by, for example, a CVD method, a sputtering method, or the like, using silicon oxide, gallium oxide, aluminum oxide, silicon nitride, silicon oxynitride, aluminum oxynitride, silicon nitride oxide, or the like. With the use of the above-described silicon nitride film or aluminum oxide film as the base insulating film 104, an impurity can be prevented from being diffused from the substrate 102 into the transistor 150. Note that the base insulating film 104 may be provided as needed.

The gate electrode 106 can be formed by a sputtering method or the like to have a single-layer structure or a stacked-layer structure using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing at least any one of these materials.

After that, the gate insulating film 108 is provided over the base insulating film 104 and the gate electrode 106 (see FIG. 4B).

The gate insulating film 108 can be formed by, for example, using silicon oxide, gallium oxide, aluminum oxide, silicon nitride, silicon oxynitride, aluminum oxynitride, silicon nitride oxide, or the like. The thickness of the gate insulating film 108 can be, for example, greater than or equal to 10 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 300 nm.

It is preferable that the gate insulating film 108 contain oxygen in a portion which is in contact with the oxide semiconductor film 110 to be formed later. In particular, the gate insulating film 108 preferably contains oxygen at an amount which exceeds at least the stoichiometric composition. For example, in the case where silicon oxide is used for the gate insulating film 108, the composition formula is preferably SiO_(2+α) (α>0). In this embodiment, silicon oxide of SiO_(2+α) (α>0) is used for the gate insulating film 108. With the use of the silicon oxide for the gate insulating film 108, oxygen can be supplied to the oxide semiconductor film 110 to be formed later and the oxide semiconductor film 110 can have excellent electrical characteristics.

The gate insulating film 108 can be formed using a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0, y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide. By using such a material, gate leakage current can be reduced. Further, the gate insulating film 108 may have a single-layer structure or a stacked-layer structure.

Next, heat treatment may be performed on the substrate 102 provided with the gate insulating film 108.

The heat treatment can be performed using an electric furnace or a device for heating an object by heat conduction or heat radiation from a heating element such as a resistance heating element. The heat treatment apparatus described in “Detailed Description of Oxide Semiconductor Film 110” can be used as appropriate. As the high-temperature gas in the GRTA apparatus, an inert gas which does not react by a heat treatment with an object to be processed, such as nitrogen or a rare gas like argon, is used. Alternatively, oxygen may be used as another high-temperature gas. When oxygen is used, release of oxygen from the gate insulating film 108 can be inhibited or supply of oxygen to the gate insulating film 108 can be performed.

In the case where the mother glass is used as the substrate 102, high process temperature and a long period of process time drastically shrink the mother glass; therefore, the process temperature of the heat treatment is preferably higher than or equal to 200° C. and lower than or equal to 450° C., more preferably higher than or equal to 250° C. and lower than or equal to 350° C.

An impurity such as water or hydrogen in the gate insulating film 108 can be removed by the heat treatment. Further, by the heat treatment, the defect density in the gate insulating film 108 can be reduced. The reduction of the impurity such as hydrogen or water in the gate insulating film 108 or the defect density in the film leads to improvement in reliability of the semiconductor device. For example, degradation of the semiconductor device during a negative bias stress test with light irradiation, which is one of the reliability tests for semiconductor devices, can be suppressed.

The heat treatment may be performed as pretreatment for formation of the oxide semiconductor film 110 to be formed later. For example, after the gate insulating film 108 is formed, heat treatment is performed in a vacuum in a preheating chamber of a sputtering apparatus and the oxide semiconductor film 110 is then formed.

Furthermore, the heat treatment may be performed more than once. For example, after the gate insulating film 108 is formed, heat treatment is performed in a nitrogen atmosphere with an electric furnace or the like; then, after heat treatment is performed in a vacuum in a preheating chamber of a sputtering apparatus, the oxide semiconductor film 110 is then formed.

Next, an oxide semiconductor film is formed over the gate insulating film 108, and a photolithography process and an etching process are performed; thus, the semiconductor film 110 that is isolated by element separation is formed (see FIG. 4B).

The detailed description of the oxide semiconductor film 110, a method for manufacturing the oxide semiconductor film 110, and the like is omitted here since it is already mentioned in “Detailed Description of Oxide Semiconductor Film 110”.

A conductive film is formed over the gate insulating film 108 and the oxide semiconductor film 110 and is subjected to a photolithography process and an etching process, whereby the source electrode 114 a and the drain electrode 114 b which are electrically connected to the oxide semiconductor film 110 are formed. At this stage, the transistor 150 is formed (see FIG. 4B).

As the conductive film used for the source electrode 114 a and the drain electrode 114 b, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any of the above elements as a component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used to have a single-layer structure or a stacked-layer structure. A metal film having a high melting point such as Ti, Mo, W, or the like or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, and a tungsten nitride film) may be stacked on one of or both of a lower side or an upper side of a metal film of Al, Cu, or the like.

Next, the interlayer insulating film 116 and the planarization insulating film (not shown) are formed over the transistor 150 (see FIG. 4C).

The interlayer insulating film 116 can be formed using a material and a method which are the same or substantially the same those of the gate insulating film 108.

For the planarization insulating film, for example, an organic resin material such as a polyimide-based resin, an acrylic-based resin, or a benzocyclobutene-based resin can be used. With the planarization insulating film, the surface of the transistor 150 can be flat.

Further, a conductive film (not shown) may be formed over the planarization insulating film. For the conductive film, a conductive material with a light-transmitting property, for example, indium tin oxide (ITO), indium zinc oxide, or the like can be used. Note that the material of the conductive film is not limited to the above. For example, a metal film (a film of aluminum, titanium, or the like) may be used. Such a metal film is preferably used because the transistor 150 can be shielded from light.

The conductive film also has a function of shielding the transistor 150 from static charges (what is called an electrostatic discharge: ESD). With the conductive film over the transistor 150, charge due to electrostatic discharge (ESD) or the like can be dissipated.

Embodiment 2 A Structure in which the Width of a Gate Electrode is Smaller than the Width of Each of a Source Electrode and a Drain Electrode

FIG. 5A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 5B is a cross-sectional view taken along line A-B in FIG. 5A; and FIG. 5C is a cross-sectional view taken along line C-D in FIG. 5A.

The semiconductor device in this embodiment has the same structure as that in Embodiment 1 except for the following points.

In the transistor 150 described in Embodiment 1, the width of the gate electrode is larger than the width of each of the source electrode and the drain electrode, whereas in a transistor 151 in this embodiment, the width of the gate electrode 106 is smaller than the width of each of the source electrode 114 a and the drain electrode 114 b.

In other words, the oxide semiconductor film 110 shown in FIGS. 5A to 5C has an end region 306 which overlaps with the gate electrode 106, and the end region 306 which overlaps with the gate electrode 106 entirely overlaps with the source electrode 114 a. Note that in this embodiment, the end region 306 entirely overlaps with the source electrode 114 a; however, the end region 306 may entirely overlap with the drain electrode 114 b.

Advantageous effects similar to those obtained in Embodiment 1 can also be obtained in this embodiment.

In particular, as shown in FIG. 5A, a leakage path due to current flowing through the one end 314 a of the first side surface 214 a of the source electrode 114 a, the first region 206 a of the end region 206 which is the nearest to the one end 314 a, the second region 206 b of the end region 206 which is the nearest to the one end 314 b, and the one end 314 b of the second side surface 214 b of the drain electrode 114 b is not formed. Similarly, a leakage path due to current flowing through the other end 414 a of the first side surface 214 a of the source electrode 114 a, the third region 206 c of the end region 206 which is the nearest to the other end 414 a, the fourth region 206 d of the end region 206 which is the nearest to the other end 414 b, and the other end 414 b of the second side surface 214 b of the drain electrode 114 b is not formed. Thus, generation of a parasitic channel in the end region 206 can be suppressed, so that formation of a parasitic transistor can be prevented.

The method for manufacturing the semiconductor device shown in FIGS. 5A to 5C is the same as that described in Embodiment 1; thus the description thereof is omitted.

Embodiment 3 Oxide Semiconductor Film Having Circular Planar Shape

FIG. 6A is a plan view of a semiconductor device according to one embodiment of the present invention; FIG. 6B is a cross-sectional view taken along line A-B in FIG. 6A; and FIG. 6C is a cross-sectional view taken along line C-D in FIG. 6A.

The semiconductor device in this embodiment has the same structure as that in Embodiment 1 except for the following points.

In a transistor 152 in this embodiment, as shown in FIG. 6A, the oxide semiconductor film 110 has a circular planar shape. The gate electrode 106 has a circular portion concentric with a circle of the oxide semiconductor film 110, and the circular portion overlaps with the oxide semiconductor film 110. The oxide semiconductor film 110 has a region which overlaps with the gate electrode 106 arranged so as to extend from one side of the circular portion of the oxide semiconductor film 110. Further, the end region 206 of the oxide semiconductor film 110 which does not overlap with the gate electrode 106 is a region located on the side surface of the circular portion except for the region where the oxide semiconductor film 110 overlaps with the gate electrode 106. The oxide semiconductor film 110 may have an elliptical shape or a polygonal shape. The region of the gate electrode 106 which overlaps with the oxide semiconductor film 110 may have a shape approximately the same as the oxide semiconductor film 110.

The planar shape of a portion of the source electrode 114 a which faces the drain electrode 114 b is U-shaped. The outline of the planar shape of a portion of the drain electrode 114 b which faces the source electrode 114 a is curved. Note that the portion of the source electrode 114 a which faces the drain electrode 114 b may have, for example, a C shape, a V shape, or the like as long as one of the source electrode and the drain electrode surrounds the end of the other with a certain distance therebetween.

Advantageous effects similar to those obtained in Embodiment 1 can also be obtained in this embodiment.

In particular, as shown in FIG. 6A, a leakage path due to the flow of current between the one end 314 a of the first side surface 214 a of the source electrode 114 a and the first region 206 a of the end region 206 which is the nearest to the one end 314 a and between the one end 314 b of the second side surface 214 b of the drain electrode 114 b and the second region 206 b of the end region 206 which is in the vicinity of the one end 314 b is not formed. Similarly, a leakage path due to the flow of current between the other end 414 a of the first side surface 214 a of the source electrode 114 a and the third region 206 c of the end region 206 which is in the vicinity of the other end 414 a and between the other end 414 b of the second side surface 214 b of the drain electrode 114 b and the fourth region 206 d of the end region 206 which is the nearest to the other end 414 b is not formed. Thus, generation of a parasitic channel in the end region 206 can be suppressed, so that formation of a parasitic transistor can be prevented.

The method for manufacturing the semiconductor device shown in FIGS. 6A to 6C is the same as that described in Embodiment 1; thus the description thereof is omitted.

<Simulation Results>

Description is made on simulations performed on transistor characteristics at the time when an end region of the oxide semiconductor film has become n-type.

FIG. 7A is a plan view of a transistor that is a simulation object. FIG. 7B is a cross-sectional view taken along line A-A′ shown in FIG. 7A. FIG. 7C is a cross-sectional view taken along line C-C′ shown in FIG. 7A.

As shown in FIGS. 7A to 7C, a gate insulating film 108 a is formed over the gate electrode 106 a, and an oxide semiconductor film 110 a using IGZO as an active layer is formed over the gate insulating film 108 a. A source electrode 114 c and a drain electrode 114 d are formed over the oxide semiconductor film 110 a.

The width of a side of the oxide semiconductor film 110 a, which extends beyond the source electrode 114 c or the drain electrode 114 d, is denoted by d; the thickness of the oxide semiconductor film 110 a is set to 20 nm; and the thickness of the gate insulating film 108 a is set to 100 nm.

FIG. 8A shows results of a first simulation. The first simulation was performed in the following conditions: the channel length L, the gate width W, and the extended width d were fixed to 6 μm, 50 μm, and 1 μm, respectively; the carrier density in the entire end region of the oxide semiconductor film 110 a was changed in the range of 1×10¹⁷ cm⁻³ to 1×10¹⁹ cm⁻³; and the drain voltage Vd was set to 1V. In FIG. 8A, the broken line, the thin solid line, and the thick solid line represent Id−Vg curves with 1×10¹⁷ cm³, 1×10¹⁸ cm³, and 1×10¹⁹ cm⁻³, respectively, as the carrier densities in the end region.

FIG. 8B shows results of a second simulation. The second simulation was performed in the following condition: the gate length L, the gate width W, and the carrier density in the end region of the oxide semiconductor film 110 a were fixed to 6 μm, 50 μm, and 1×10¹⁷ cm⁻³, respectively; the extended width d was changed in the range of 0 to 3 μm; and the drain voltage Vd was set to 1V. In FIG. 8B, the broken line, the thin solid line, and the thick solid line represent Id−Vg curves at the time when the extended widths dare 3 μm, 1 μm, and 0 μm, respectively.

As shown in FIG. 8A, the higher the carrier density at the end region of the oxide semiconductor film 110 a, the larger the hump generated in the Id−Vg curve. Further, as shown in FIG. 8B, the larger the extended width d, the smaller the hump generated in the Id−Vg curves.

The above simulation results show that generation of a parasitic channel in the end region of the oxide semiconductor film 110 a can be suppressed by reducing the carrier density in the end region of the oxide semiconductor film 110 a, that is, by suppressing the change of the end region of the oxide semiconductor film 110 into n-type, and thus the generation of a hump in the Id−Vg curve can be suppressed. Further, the above simulation results show that generation of a hump in the Id−Vg curve can be suppressed by positioning the end region apart from the channel region of the oxide semiconductor film 110 a.

Embodiment 4 Liquid Crystal Display Device

FIG. 9 is a top view illustrating a pixel of a liquid crystal display device of one embodiment of the present invention. FIG. 10 is a circuit diagram of the pixel of the liquid crystal display device shown in FIG. 9.

As shown in FIGS. 9 and 10, the liquid crystal display device includes a pixel 160, and the pixel 160 includes the transistor 150 shown in FIGS. 1A to 1B. Note that the transistor 150 shown in FIGS. 1A to 1C is used in this embodiment; however, the transistor 151 shown in FIGS. 5A to 5C or the transistor 152 shown in FIGS. 6A to 6C may be used.

The circuit diagram of the pixel 160 of the liquid crystal display device shown in FIG. 9 will be described with reference to FIG. 10. A gate electrode of the transistor 150 is connected to a scanning line 161, one of a source electrode and a drain electrode of the transistor 150 is connected to a signal line 162, and the other of the source electrode and the drain electrode is connected to a capacitor 166 and a liquid crystal element 167.

The pixel 160 will be described in detail with reference to FIG. 9. The gate electrode 106 of the transistor 150 is connected to the scanning line 161; the source electrode 114 a of the transistor 150 is connected to the signal line 162; and the drain electrode 114 b of the transistor 150 is connected to a liquid crystal element and the capacitor 166. Specifically, the drain electrode 114 b of the transistor 150 is connected to a capacitor wiring 163, and is electrically connected to a pixel electrode 164 in a contact hole 165 through the capacitor wiring 163. The liquid crystal element 167 in FIG. 10 includes the pixel electrode 164, and a liquid crystal material and a counter electrode which are not shown in the figure. In FIG. 9, the gate electrode 106 and the scanning line 161 are formed using the same layer and connected to each other; however, they may be formed using different layers and connected to each other, the source electrode 114 a and the signal line 162 are formed using the same layer and connected to each other; however, they may be formed using different layers and connected to each other, and the drain electrode 114 b and the capacitor wiring 163 are formed using the same layer and are connected to each other; however, they may be formed using different layers and connected to each other.

According to this embodiment, with the use of the transistor 150 shown in FIGS. 1A to 1C, the adverse effects that voltage in the pixel 160 of the liquid crystal display device cannot be retained, current consumption is increased, and the like can be reduced.

Embodiment 5 EL Display Device

FIG. 11 is a top view illustrating a pixel of an EL display device of one embodiment of the present invention. FIG. 12 is a circuit diagram of the pixel of the EL display device shown in FIG. 11.

As shown in FIG. 11 and FIG. 12, the EL display device includes a pixel 170, and the pixel 170 includes a selection transistor 150 a and a driving transistor 150 b. The transistor 150 shown in FIGS. 1A to 1C are used as each of the selection transistor 150 a and the driving transistor 150 b. Note that in this embodiment, the transistor 150 shown in FIGS. 1A to 1C is used as each of the selection transistor 150 a and the driving transistor 150 b; however, the transistor 151 shown in FIGS. 5A to 5C or the transistor 152 shown in FIGS. 6A to 6C may be used.

The circuit diagram of the pixel 170 of the EL display device shown in FIG. 11 will be described with reference to FIG. 12. A gate electrode of the selection transistor 150 a is connected to a scanning line 171; one of a source electrode and a drain electrode of the selection transistor 150 a is connected to a signal line 172; the other of the source electrode and the drain electrode of the selection transistor 150 a is connected to a gate electrode of the driving transistor 150 b and one electrode of a capacitor 176. One of a source electrode and a drain electrode of the driving transistor 150 b is connected to a power supply line 174, and the other of the source electrode and the drain electrode of the driving transistor 150 b is connected to a light-emitting element 178. The other electrode of the capacitor 176 is connected to the power supply line 174.

Next, the pixel 170 will be described in detail with reference to FIG. 11. The gate electrode 106 of the selection transistor 150 a is connected to the scanning line 171, and the source electrode 114 a of the selection transistor 150 a is connected to the signal line 172. The drain electrode 114 b of the selection transistor 150 a is electrically connected to one capacitor electrode 173 a of the capacitor 176 through a contact hole 175, and the capacitor electrode 173 a is connected to a gate electrode 206 of the driving transistor 150 b. An oxide semiconductor film 210 is provided over the gate electrode 206 with the gate insulating film interposed therebetween. Part of the power supply line 174 serves as the other capacitor electrode of the capacitor 176, and the power supply line 174 is connected to a drain electrode 204 b of the driving transistor 150 b. A source electrode 204 a of the driving transistor 150 b is electrically connected to a first electrode 177 of the light-emitting element through a contact hole 179. The light-emitting element 178 shown in FIG. 12 includes the first electrode 177, and a light-emitting layer and the second electrode which are not shown in the figure. Note that in FIG. 11, the gate electrode 106 and the scanning line 171 are formed using the same layer and connected to each other; however, they may be formed using different layers and connected to each other, the source electrode 114 a and the signal line 172 are formed using the same layer and connected to each other; however, they may be formed using different layers and connected to each other, and the capacitor electrode 173 a and the gate electrode 206 are formed using the same layer and are connected to each other; however, they may be formed using different layers and connected to each other.

According to this embodiment, with the use of the transistor 150 shown in FIGS. 1A to 1C, the adverse effects that voltage in the pixel 170 of the EL display device cannot be retained, current consumption is increased, and the like can be reduced

This application is based on Japanese Patent Application serial no. 2012-093512 filed with Japan Patent Office on Apr. 17, 2012, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a gate electrode; a source electrode and a drain electrode; and an oxide semiconductor film including: a channel region between a first side surface of the source electrode and a second side surface of the drain electrode which is opposite to the first side surface; a first region that is the nearest to one end of the first side surface; a second region that is the nearest to one end of the second side surface; a first end region which is located between the first region and the second region and which does not overlap with the gate electrode; a third region that is the nearest to the other end of the first side surface; a fourth region that is the nearest to the other end of the second side surface; and a second end region which is located between the third region and the fourth region and does not overlap with the gate electrode.
 2. The semiconductor device according to claim 1, wherein the oxide semiconductor film is located over the gate electrode, and wherein the source electrode and the drain electrode are over and in contact with the oxide semiconductor film.
 3. The semiconductor device according to claim 1, wherein one of the source electrode and the drain electrode and the gate electrode are each arranged so as to extend from one side of the oxide semiconductor film, and wherein the one of the source electrode and the drain electrode overlaps with both the oxide semiconductor film and the gate electrode.
 4. The semiconductor device according to claim 1, wherein a width of the gate electrode is larger than a width of the one of the source electrode and the drain electrode in a direction of a channel width.
 5. The semiconductor device according to claim 1, wherein the oxide semiconductor film is a film containing an oxide of one or more elements selected from indium, zinc, gallium, and tin.
 6. The semiconductor device according to claim 1, wherein the oxide semiconductor film includes a crystal portion where a c-axis is aligned in a direction parallel to a normal vector of a surface where the oxide semiconductor film is formed.
 7. A semiconductor device comprising: a gate electrode; a source electrode and a drain electrode; and an oxide semiconductor film including: a channel region between a first side surface of the source electrode and a second side surface of the drain electrode which is opposite to the first side surface; a first region that is the nearest to one end of the first side surface; a second region that is the nearest to one end of the second side surface which is the nearest to the one end of the first side surface; a first end region which is located between the first region and the second region and which does not overlap with the gate electrode; a third region that is the nearest to the other end of the first side surface; a fourth region that is the nearest to the other end of the second side surface; and a second end region which is located between the third region and the fourth region and does not overlap with the gate electrode.
 8. The semiconductor device according to claim 7, wherein the oxide semiconductor film is located over the gate electrode, and wherein the source electrode and the drain electrode are over and in contact with the oxide semiconductor film.
 9. The semiconductor device according to claim 7, wherein the gate electrode and one of the source electrode and the drain electrode are each arranged so as to extend from one side of the oxide semiconductor film, and wherein the one of the source electrode and the drain electrode overlaps with both the oxide semiconductor film and the gate electrode.
 10. The semiconductor device according to claim 7, wherein the first end region is a continuous region from the first region to the second region.
 11. The semiconductor device according to claim 7, wherein the second end region is a continuous region from the third region to the fourth region.
 12. The semiconductor device according to claim 7, wherein the first end region is provided outside a space between the first side surface and the second side surface.
 13. The semiconductor device according to claim 7, wherein the oxide semiconductor film has a third end region which overlaps with the gate electrode, and wherein the third end region entirely overlaps with at least one of the source electrode and the drain electrode.
 14. The semiconductor device according to claim 7, wherein the oxide semiconductor film is a film containing an oxide of one or more elements selected from indium, zinc, gallium, and tin.
 15. The semiconductor device according to claim 7, wherein the oxide semiconductor film includes a crystal portion where a c-axis is aligned in a direction parallel to a normal vector of a surface where the oxide semiconductor film is formed.
 16. The semiconductor device according to claim 1, wherein the first end region is a continuous region from the first region to the second region.
 17. The semiconductor device according to claim 1, wherein the second end region is a continuous region from the third region to the fourth region.
 18. The semiconductor device according to claim 1, wherein the first end region is provided outside a space between the first side surface and the second side surface.
 19. The semiconductor device according to claim 1, wherein the oxide semiconductor film has a third end region which overlaps with the gate electrode, and wherein the third end region entirely overlaps with at least one of the source electrode and the drain electrode. 